Medial temporal lobe activity can distinguish between old and new stimuli independently of overt behavioral choice

We collected fMRI data and confidence ratings as participants performed a recognition memory task that intermixed recently studied words and new (non-studied) words. We first replicated a typical finding from such studies; namely, increasing activity in medial temporal lobe structures with increasing confidence in the old/new decision. Because there are greater proportions of old items at higher confidence levels, such activity could be related to the confidence ratings or to whether items are old or new. When activity associated with old and new items was analyzed separately, we found that activity in the hippocampus bilaterally, as well as in anterior parahippocampal gyrus, was associated with the actual old/new status of the items rather than to which items participants believed to be old. Accordingly, activity in the medial temporal lobe can be modulated by the old/new status of stimuli and does not always track the behavioral response.